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Failure mechanisms in denture adhesives Yiran An a,b , Danyang Li a,b , Nima Roohpour c , Julien E. Gautrot a,b,∗ , Asa H. Barber a,b,d,∗ a

Institute of Bioengineering, University of London, Mile End Road, London E1 4NS, UK School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, London E1 4NS, UK c Consumer Healthcare R&D, GlaxoSmithKlein, St George’s Ave, Weybridge KT13 0DE, UK d School of Engineering, University of Portsmouth, Portsmouth PO1 3DJ, UK b

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Article history:

Objective. The mechanical properties of bio adhesives in oral care application are expected

Received 2 January 2016

to be critical in defining the stability and release of devices such as dentures from the oral

Accepted 18 January 2016

tissue. A multiscale experimental mechanical approach is used to evaluate the performance

Available online xxx

of denture adhesive materials.

Keywords:

rating adhesive material from a representative polymethyl methacrylate (PMMA) surface

Methods. The inherent mechanical behavior of denture fixatives was examined by sepaDenture

using atomic force microscopy (AFM) approaches and compared to macroscopic mechanical

Dental materials

testing.

Adhesion

Results. Failure of denture adhesive material was found to be critically dependent on the for-

Failure

mation of fibrillar structures within the adhesive. Small scale mechanical testing provided

PMMA

evidence for the mechanical properties of the fibrillar structures formed within the adhe-

Mechanical properties

sive in macroscopic mechanical testing and indicated the importance of the forces required to fail the adhesive at these small length scales in controlling both the maximum forces

AFM

sustained by the bulk material as well as the ease of separating the adhesive from PMMA surfaces. Significance. Our results are important in defining the performance of denture fixative materials and their control of adhesive behavior, allowing the potential to tune properties required in the adhesion and removal of dentures. © 2016 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Complex processes regulate the adhesion of biomaterials to tissues and other interfaces [1–5], with the magnitude of such interactions defining the overall performance of implants. In particular, understanding the mechanical properties of the adhesive at the interface with the device and tissue are required for evaluation of resultant adhesive performance



[5–7]. Adhesives for dentures are particularly demanding and need to provide fixation of the denture within the aggressive environment of the oral cavity but allow relatively effective removal on demand [8–13]. The adhesion of dentures is almost contradictory as both high adhesion for fixing and low adhesion for ease of removal are required. The potential sensitivity of adhesion in controlling denture fixing and removal motivates the need for techniques that are able to comprehensively evaluate the adhesion process. The relationship

Corresponding authors at: Institute of Bioengineering, Queen Mary University of London, Mile End Road, London E1 4NS, UK. E-mail addresses: [email protected] (J.E. Gautrot), [email protected] (A.H. Barber).

http://dx.doi.org/10.1016/j.dental.2016.01.007 0109-5641/© 2016 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: http://dx.doi.org/10.1016/j.dental.2016.01.007

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between molecular interactions at interfaces and in the bulk of the adhesive involves evaluation of stress transfer and failure mechanisms that are currently poorly defined. Methods that quantify structure–property relationships controlling the behavior of such interfaces are important in understanding and designing implants and adhesives in the biomedical field, including for oral care applications. From fundamental considerations, electrostatic interactions and hydrogen bonding are known to significantly contribute to the bulk mechanical and rheological properties of biomaterials used for dental adhesive applications [14]. However, the role of these interactions, together with other hydrophobic interactions occurring at the surface of implants and denture, on the failure of adhesive remains unclear. Chemical design of biomaterials is therefore important in controlling the failure of the adhesive, specifically at interfaces or within the bulk, and enables tailoring of the mechanical properties of the adhesive to function. The location of failure occurs either at an interface (adhesive failure) or in the bulk of the adhesive (cohesive failure) and has been shown to be particularly important in defining resultant adhesive performance [15,16]. Suitable experimental techniques are required to both measure the mechanical properties of the dental adhesive directly and relate to the chemistry of the adhesive. Microscale mechanical testing using atomic force microscopy (AFM) is often employed to understand these mechanical properties directly and, as the size considered is relatively small, geometric considerations that dominate at larger lengths can be ignored so that the inherent material chemistry is probed [17–21]. Extension of small-scale mechanical testing has incorporated in situ imaging using scanning electron microscopy (SEM) that allows correlation between the mechanical response of a biomaterial and the observed deformation or failure event, the latter being important in defining either adhesive or cohesive failure [17–20]. The powerful combination of small-scale mechanical testing and in situ imaging is therefore applicable to denture adhesives to provide quantitative evidence of the influence of chemistry on failure mechanisms. In this work, the mechanical properties of a dental adhesive in contact with poly(methyl methacrylate) (PMMA) dentures was examined. Our approaches aim to correlate the larger macroscopic length scale to more fundamental microscale behavior for comprehensive structure–property relationships.

2.

Materials and methods

2.1.

Overview

Commercially available dental adhesives (GSK, UK), Poligrip® , Ultra Wernets® , Denture Fixative Powder (PDFP) and Poligrip® , Ooze-Control Tip® Denture Adhesive Cream (PDAC), were used in this study. PDFP is composed of poly(methylvinylether/maleic acid) sodium–calcium mixed partial salt, cellulose gum and aroma while PDAC is composed of poly(methylvinylether/maleic acid) sodium–magnesium– zinc mixed partial salt, petrolatum, cellulose gum, mineral oil, silica, poly(methylvinylether/maleic acid), flavor, Red 30 aluminum lake and Red 7 calcium lake. PDAC contains more hydrophobic compounds such as hydrocarbon vehicles Please cite this article in press as: http://dx.doi.org/10.1016/j.dental.2016.01.007

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(mineral oil and petrolautum), in addition to MVE/MA copolymer. These compounds may affect the hydration of the polymers and gel formation resulting in different adhesion behavior. Both materials were applied as adhesives to investigate their adhesion behavior with a PMMA substrate representing a standard denture material. Both PDFP and PDAC were wet by mixing with distilled water at the ratio of 1:1 in a petri dish before mechanical testing. This approach was considered to represent the hydration state of the adhesives in typical usage conditions in the oral cavity for fixing dentures [22]. Adhesion behavior of PDFP and PDAC with PMMA was investigated at both the macroscale and microscale to fully characterize their adhesion mechanics, as shown schematically in Fig. 1.

2.2.

Macroscopic testing

Macroscopic testing was performed by detaching two adhesive-bonded PMMA plates and examining their adhesion properties. Commercial PMMA plates were cut using a circular saw (Struers, Germany) into dimensions of 3 mm × 6 mm × 15 mm with the cross section area of 3 mm × 6 mm for adhesive attachment. Two PMMA plates were bonded with a small amount (weighted approximately 0.15 ± 0.5 g) of wet adhesive sufficient to fully cover the cross section area and held together by hand for 1 min before mounting the two adhesive-bonded PMMA plates on a commercial microtester (Deben, 200 N tensile stage, U.K., as shown in Fig. 1. The microtester was mounted onto a scanning electron microscope (SEM) sample stage within the SEM chamber (Quanta 3D FEG, FEI, EU/USA) so that mechanical testing was observed using the SEM. The opposite ends of each PMMA plate were clamped tightly by the sample grip of the microtester, leaving the two adhesive-bonded cross section surfaces in the middle of the gauge. Initial distance between the two sample grips was calibrated to 10.09 mm. Uniaxial tensile test was performed by translating one of the grips away from the other at a constant rate of 0.5 mm min−1 , causing the two adhesive-bonded cross section surfaces to detach. The force and extension applied to the sample was recorded using the microtester while SEM allowed physical deformation to be related to the mechanical information.

2.3.

Nanoscale AFM testing

2.3.1.

Sample preparation

Adhesion behavior between PDFP/PDAC and PMMA was further investigated at smaller length scales to evaluate the relationship between mechanical properties and interfacial chemistry in a geometrically simple setup. Spherical microscale PMMA-coated silica beads were used to study the PDFP/PDAC-PMMA adhesion at the microscale. These microscale experiments were important and allowed comparison with macroscopic testing that incorporates chemistry as well as potentially larger structural features, such as voids, that may dominate adhesion behavior. The microbeads were prepared by coating commercial 3.43 ␮m diameter silica beads (Bangs Laboratories Inc., USA) with PMMA polymer brushes using protocols adapted from the literature [23,24]. 1 ml of toluene kept under nitrogen was added to 50 mg silica beads and sonicated for 10 min until the suspension was cloudy. The

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Fig. 1 – Schematic showing the macroscale and microscale tests performed to evaluate the performance of PDFP/PDAC adhesives using (top) a commercial microtester and (bottom) an AFM setup.

bead suspension was then centrifuged at 5000 rpm for 30 s and the toluene allowed to aspire. The sonication and centrifugation process was repeated three times and the beads were finally dispersed in 1 ml toluene. The grafting of the initiator silane monolayer for atom transfer radical polymerization was carried by adding 50 ␮l of Et3 N and 10 ␮l silane initiator to the 1 ml silica dispersion followed by shaking overnight. The silica beads with silane initiator were then washed with 1 ml DMF three times, stored in 1 ml DMF and transferred to a refrigerator held at 4 ◦ C until polymerization. 1 ml DMF and 0.5 ml deionized water was added into the 1 ml silica beads dispersion in the reaction vessel, and then degased for 30 min with argon bubbling. The monomer solution of bipyridine (167 mg), Cu(II)Br (97 mg), MMA (4.5 ml), deionized water (2 ml), DMF (8 ml) was degased via argon bubbling for 30 min while stirring, then Cu(I)Br (62 mg) was quickly added into the solution, followed by another 30 min bubbling. A further 2.5 ml of monomer solution was added to the reaction vessels containing 2.5 ml of the particle suspension. Polymerization was allowed to proceed for 2 h under argon at room temperature. To terminate the polymerization, the reaction mixture was bubbled with compressed air until a blue coloration was observed. The obtained SiO2 -PMMA suspension was centrifuged and washed three times with deionized water and DMF (v/v = 1:4) to remove the catalyst and suspended PMMA polymer, during which, sonication was applied to reduce the aggregation. Finally the PMMA-coated beads were dispersed in 1 ml deionized water and stored in a refrigerator before usage. The diameter of the PMMA-coated bead was ∼3.53 ␮m, highlighting an increased silica bead diameter due to the coating, as measure using SEM.

2.3.2.

AFM mechanical testing

A small volume of PMMA-coated bead solution was deposited onto a silicon wafer and left overnight to allow solvent evaporation prior to mechanical testing. This step provided a sparse Please cite this article in press as: http://dx.doi.org/10.1016/j.dental.2016.01.007

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distribution of beads over the silicon wafer surface. A custom built AFM (Attocube GmbH, Germany) integrated within an SEM (Quanta 3D FEG, FEI, EU/USA), as described in previous work [19], was used to attach individual beads to the apex of an AFM tip. The combination of SEM and AFM is effective as the AFM provides high-resolution force information while the SEM gives imaging capabilities. A schematic of the combined AFM–SEM set-up is shown in Fig. 1 and highlights the experimental setup where the AFM tip contacts an individual bead on the silicon wafer surface. Pickup of an individual PMMA-coated bead to the apex of the AFM tip was achieved by first translating the apex of the AFM tip into a droplet of glue (SEMGLU, Kleindiek Nanotechnik GmbH, Germany). Removal of the AFM tip from the glue caused deposition of a small amount of glue at the apex of the AFM tip. The AFM tip was subsequently moved toward the PMMA-coated beads on the silicon wafer. The beads were confirmed as being dispersed over the silicon wafer surface as shown in Fig. 2a. The AFM tip was translated into contact with an individual bead so that the bead was fixed to the glue at the AFM tip apex as shown in Fig. 2b. The high-vacuum compatible adhesive glue hardens under electron beam irradiation. Relatively low imaging electron currents of 93 pA at 10 kV were used so that the glue remains uncured and deformable. Focusing a high current electron beam of 1.5 nA for approximately 10 min on a small area of the glue causes initiation of glue polymerization and subsequent solidification. In this way, the glue solidifies and attaches the bead firmly to the apex of the AFM tip. Fig. 2c shows a SEM micrograph of an individual PMMA-coated bead attached to the apex of the AFM tip. The PMMA-coated bead attached to the AFM tip was retained in the AFM setup and wet PDFP and PDAC samples were mounted to the AFM sample stage for adhesion testing. Each adhesive was translated by the piezo positioners of the AFM system until contact with an individual PMMA-coated bead at the apex of the AFM tip and the PDFP or PDAC adhesive

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Fig. 2 – (a) SEM micrographs showing (a) the PMMA-coated beads dispersed on the silicon wafer, (b) contact of the AFM tip to one PMMA-coated bead and (c) an individual PMMA-coated bead attached to the apex of the AFM tip. The dimension of the bead was confirmed as 3.53 ␮m.

surface was achieved. After contact, the AFM tip was retracted away from the sample at a constant rate of 0.2 ␮m s−1 so that interfacial failure and complete separation of the bead from the adhesive occurred. A soft AFM cantilever with a spring constant of 0.13 N m−1 , measured using the Sader calibration method [25], was used in this work to provide sufficient force resolution.

2.4.

Compositional study using EDS

X-ray energy dispersive spectroscopy (EDS) microanalysis within an SEM (Inspect SEM, FEI Company, EU/USA) was used to investigate the chemical composition of the PMMA crosssectional surface after failure of the adhesive performed using the Deben microtester. The EDS detection was performed under 10 keV at the working distance of 10 mm. The depth of the layer under analysis with EDS is dependent on the sample and the beam energy of the electron beam, calculated as 5.5 ␮m using Monte Carlo simulation (Casino v2.42, Can.). Chemical analysis allows understanding of the failure mechanism at the adhesive surface as many adhesives, including those used in this study, contain inorganic ions or fillers. Thus, analysis of a PMMA surface after failure can reveal if inorganics are present, suggesting failure occurred within the adhesive so that a thin adhesive layer is present at the PMMA surface, or a clean PMMA surface indicating PMMA-adhesive interfacial failure.

3.

Results and discussion

3.1.

Macroscopic testing

3.1.1.

PDFP

Fig. 3 shows a typical force-extension curve of the adhesive recorded during the macroscopic test using PDFP as the adhesive between the two PMMA plates. The plot shows an initial steep slope until a maximum force of ∼1.34 N at an extension of ∼4 ␮m was reached, followed by a long yielding phase, associated with fast stress relaxation and high ultimate strains to the maximum adhesive extension of ∼371 ␮m. The yield point A shown in Fig. 3 is a critical transition point between two regions and was correlated with the adhesive behavior observed using SEM. Please cite this article in press as: http://dx.doi.org/10.1016/j.dental.2016.01.007

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Fig. 3 – Typical force-extension curve recorded during the separation of two PMMA plates adhered together by PDFP.

SEM micrographs of the PDFP adhesive at yielding (point A) are shown in Fig. 4. An adhesive interface between the two PMMA plates was observed in Fig. 4 due to clean PMMA surfaces devoid of polymeric residue, which is indicative adhesive delamination and is proposed to occur at the yielding point. The formation of fibrils in the adhesive clearly shown in Fig. 4d is also observed at this yielding stage. The adhesion behavior of the PDFP adhesive during force relaxation at higher displacements, as indicated at point B in Fig. 3, was examined using SEM imaging. Fig. 5 indicates further necking of the adhesive fibrils that formed at the yielding point A associated with a reduced contact area between the PMMA plates. Detachment of the adhesive from the PMMA surface or failure within the adhesive is critical in understanding the adhesive performance and requires chemical evaluation of the evolved surfaces. EDS of sample provided chemical composition of the failed PMMA-adhesive interface. Fig. 6 shows scanning electron micrographs of two PDFP-bound samples taken to failure. Areas A and B in Fig. 6 correspond to extensions A (yielding point) and B (higher strain) in Fig. 3, respectively. These two different areas were imaged with EDS using 5 detecting points on each sample. Table 1 states

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Fig. 4 – Scanning electron micrographs of the PDFP-bonded interface at the yielding point. The images are obtained from different samples and show the delamination behavior of the PDFP adhesive, indicated by white arrows.

the measured chemical composition of the interface of both samples. These EDS results indicate the presence of areas displaying a clean PMMA plate surface, highlighted by an absence in Na and areas still coated with the adhesive. Indeed, EDS only shows carbon and oxygen elements for clean PMMA areas, whereas additional Na is detected (as the counter ion of the polyanionic adhesive component of PDFP) in other areas, indicating the presence of adhesive PDFP. This behavior was not altered as the tensile test proceeded, suggesting that isolated islands of residual adhesive are left at the surface of PMMA after failure. This data confirms that PDFP failure occurs at the interface with the PMMA, leaving areas of the PMMA surface uncovered by the adhesive. Hence, failure of PMMA surfaces bonded by PDFP is proposed as a partial delamination of adhesive from the PMMA surface, followed by necking of the adhesive itself at larger strains. In this second high strain region, local failure of the formed fibrils is expected to be associated with high strain induced polymer chain alignment around defects such as bubbles, as is commonly observed in glassy polymers and are inherent to relatively inhomogeneous materials [26].

3.1.2.

PDAC

The adhesive behavior of PDAC to PMMA surfaces was studied using the same macroscopic and microscale mechanical testing employed for PDFP. The nature of the PDAC cream, which displays a heterogeneous morphology with mixtures of hydrophilic and hydrophobic domains, is expected to alter Please cite this article in press as: http://dx.doi.org/10.1016/j.dental.2016.01.007

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bonding to the PMMA surface and subsequent failure mechanism. Specifically, the apolar groups of the PMMA are expected to interact, although weakly, with the hydrophobic domains of the PDAC. PMMA plates were bonded with a small amount of adhesive sufficient to cover the cross section area and held together for 1 min. before mounting the sample on the microtensile tester. The loading rate was kept constant at 0.5 mm min−1 for all tensile tests. A typical force-extension curve recorded during the tensile test is shown in Fig. 7 and shows an initial steep slope, followed by a long yielding phase, associated with moderate stress relaxation and high ultimate strains. PDAC shows a larger yielding force (∼6.14 N) and higher failure strain compared with PDFP. The stress relaxation rate is reduced when compared to PDFP according to the force-extension curves. Therefore, we can conclude that PDAC shows a better adhesion capability than PDFP in these conditions. Three stages of deformation were characterized by SEM during the tensile test and indicated yielding (point A in Fig. 7), stress relaxation (point B in Fig. 9) and progression to failure (point C in Fig. 9). Fig. 8(A) shows scanning electron micrographs of the yielding at point A and highlights initial necking and formation of adhesive PDAC over the cross section area. This behavior provides an initial high adhesive force between the PMMA plates. Adhesive deformation at stage B was observed using SEM and is shown in Fig. 8(B), highlighting continued necking of the adhesive fibrils that now sustain a weaker adhesive force and implies the occurrence of

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Fig. 5 – Scanning electron micrographs of the PDFP-bonded interfacial failure at higher sample strains. The images obtained show the deformation behavior of the PDFP adhesive at higher strains at a range of magnifications and associated formation of fibrils (indicated by white arrows).

Fig. 6 – SEM images of the cross-sectional area of the PMMA plate with the failed interface of adhesives stretched to extensions A and B in Fig. 5. For both samples, EDS measurements were performed at 5 different locations to probe the chemical composition of the structures imaged, with the red areas indicating the region covered with PDFP adhesive.

stress relaxation. Evidence of fibril failure within this region is observed and was found to be progress with increasing strain. As a result, the number of fibrils was considerably reduced compared to stage A, a phenomenon that underlies the gradual, stepwise, failure of PDAC-bound interfaces. Failure of the adhesive at stage C was also characterized via SEM imaging (see Fig. 8(C)) and indicated the complete failure of adhesive Please cite this article in press as: http://dx.doi.org/10.1016/j.dental.2016.01.007

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fibrils. The progressive decrease of applied forces to the sample with strain is thus clearly associated with the failure of individual fibrils created at the yielding point. To summerise, the PDAC exhibits a higher yield force and ultimate tensile strain to failure than PDFP, with SEM imaging observing the formation of the adhesive fibrils necking at all stages until ultimate failure strain for both adhesives. Hence

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Table 1 – Chemical composition (measured by EDS) of different areas for samples taken to extensions corresponding to points A and B in Fig. 3.

Area A 1 2 3 4 5 Area B 1 2 3 4 5

C

O

Na

69.93% 70.95% 80.81% 51.15% 53.39%

30.07% 29.05% 19.19% 44.48% 41.33%

0 0 0 4.37% 5.29%

77.45% 71.76% 53.55% 47.62% 79.02%

22.55% 28.24% 43.13% 44.69% 20.31%

0 0 3.33% 7.69% 0.67%

the failure mechanisms of PMMA plates bound with PDAC is through cohesive failure whereas PDFP fails predominantly through adhesive failure. Methods employing tensile testing correlated to SEM imaging, used to characterize the adhesives, suggests that mechanical properties of the hydrated adhesive thus dominate the failure of the bound interface. Whether this failure is a result of stronger interactions between PDAC and the PMMA surface or whether it arises from differences in the strength and number of fibrils formed in PDAC is not clear. Hence mechanical and adhesive testing at the nano- to micro-scale was required to elucidate the difference in failure mechanism observed between PDFP and PDAC.

3.2.

Nanoscale AFM testing

Single PMMA-coated beads fixed to AFM tips were placed in contact with the adhesives and subsequently detached using the AFM in SEM as described in Section 2.3.2. All AFM tests were performed within a short period of time (

Failure mechanisms in denture adhesives.

The mechanical properties of bio adhesives in oral care application are expected to be critical in defining the stability and release of devices such ...
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